Various forms of blinds, having slats, incorporate a worm and gear angle drive for rotating or tilting the slats. One form of such blind is the well known vertical blind. In such blinds a header rail supports a plurality of sliding carriers, and vertical blind slats hang downwardly from such carriers. The carriers are slidable along a drive rod, which can be rotated from one or other end of the track. Each of the carriers incorporates a worm and gear drive. When the drive rod is rotated the worm portion of the drive transmits movement to the gear portion of the drive which thus rotates the blind slats between open and closed positions.
In addition to providing a mechanism for rotating or tilting the slats, such blinds also provide a mechanism for the drawing or of the sliding carriers to one end or the other of the track, to open the window. Such mechanisms may incorporate a system of pull cords, or in some cases, may incorporate an elongated helical drive rod, with all of the carriers incorporating a internally helically threaded nut device, so that rotation of the helical drive rod will draw all of the carriers to one end of the rail or the other.
The components in such carriers and drives are almost invariably manufactured of thermoplastic material, and are relatively light, and, while having sufficient strength for the purpose for which they are intended, can be easily damaged if they are abused.
The operation of the drive rod is achieved either by means of a wand, or by means of a metal or plastic chain or a cord and a gear arrangement, being located at one or other end of the rail. The operation of the drive rod in a normal fashion for which it is intended will cause rotation of the blind slats between open and closed positions. However, if for some reason the drive rod is over-rotated, for example by a child, or by someone who is ignorant of the way in which such blind controls must be operated, then the plastic components in the worm and gear drives can become damaged, and the blind slats will no longer rotate.
In order to overcome this problem, manufacturers of such blinds have developed a torque limiting device or clutch, in the worm portion of the drive. The effect of this is that when the blind slat is fully rotated to a closed position, the gear portion of the drive reaches a stop, so that the blind slat cannot be rotated further. If the rod is then subject to over-rotation, the clutch in the worm portion of the drive will simply slip, and thus prevent the drive from becoming damaged.
Such devices are effective to prevent damage, and are popular. However, the components for such a torque limiting device must be fitted into a small space in each carrier. Typically, the worm portion of the drive will be no more than about 0.5 inches in diameter, and 0.25 inches in thickness. Such torque limiting devices incorporated in the worm unit are usually fabricated in two or three components, and they must fit together and occupy no more space than the worm unit without the torque limiting device. Consequently, the components of the torque limiting device are small, and must be manufactured with a relatively high degree of accuracy in order to function. As a result, existing torque limiting devices in such carriers are relatively expensive.
A further factor mitigating against the use of such torque limiting devices was the fact that as the parts became smaller, and at the same more complex, it required more and more highly skilled and highly trained personnel for the assembly of such blinds. This still further increased the expense of such blinds.
Clearly, it would be advantageous if a torque limiting device for use in such worm and gear drives could be devised which did not require several separate components since it would both reduce the cost of manufacture, and also reduce the complexity of the task of assembling the components together.
The assembly of these relatively intricate blind components and carriers is itself at best a tiresome manual task that must be carried out with great accuracy in order for the blind to function satisfactorily. One of the factors is that it is necessary to ensure that the gear portion of the drive, in the carrier, from which the individual slat is suspended, shall be oriented in the correct rotational position in each of the carriers. This is essential so that it ensures that each of the blind slats will always be suspended in a plane parallel to all of the other blind slats, so that when they are rotated to open and close, they present a uniform appearance.
It would therefore be highly advantageous if, in the design of such a torque limiting drive system, it was possible to ensure that it was "self aligning" in the sense that it could only be assembled, when the gear shafts were all in the correct rotational position.
While this discussion has dealt primarily with so-called vertical blinds, in which slats are hung from individual slidable carriers, it will be appreciated that it is also equally applicable to similar blinds when used overhead, i.e., with the slats at an angle, or horizontal, and also with minor modifications, to horizontal Venetian blinds, which also incorporate a worm and gear drive for tilting the slats.
In the past, proposals have been made to incorporate a torque limiting device in the worm and gear drive of a horizontal venetian blind. In this proposal, some of the teeth of the gear drive were simply altogether removed, so that the shaft around the arc where the teeth were removed presented a smooth generally semi-cylindrical surface. This system however was unsatisfactory. Once the gear drive had been rotated to the point where the teeth ended, there was then simply nothing at all for the worm to engage. In a sense this did in fact limit the torque, since there was no longer any torque of any kind transmitted from the work to gear drive. However, it was found that using this system, since the worm and gears were essentially disengaged at this point, it was not possible to then re-engage them and operate the worm and gear drive in the reverse direction. Consequently this system was unsatisfactory.